N-(phosphonomethyl)glycine (known in the agricultural chemical industry as glyphosate) is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in aqueous formulations. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making N-(phosphonomethyl)glycine are known in the art. Franz (U.S. Pat. No. 3,950,402) teaches that N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as PMIDA) with oxygen in the presence of a catalyst comprising a noble metal deposited on the surface of an activated carbon support:

Other by-products may also form, such as formic acid, which is formed by the oxidation of the formaldehyde by-product; aminomethylphosphonic acid (AMPA) and methyl aminomethylphosphonic acid (MAMPA), which are formed by the oxidation of N-(phosphonomethyl)glycine; and iminodiacetic acid (IDA), which is formed by the de-phosphonomethylation of PMIDA. Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal into the reaction solution (i.e., leaching) result because under the acidic conditions of the oxidation reaction, some of the noble metal is oxidized into a more soluble form and both PMIDA and N-(phosphonomethyl)glycine act as ligands that tend to further solubilize the noble metal.
In U.S. Pat. No. 3,969,398, Hershman teaches that activated carbon alone, without the presence of a noble metal, may be used to catalyze the oxidative cleavage of PMIDA to form N-(phosphonomethyl)glycine. In U.S. Pat. No. 4,624,937, Chou further teaches that the activity of the carbon catalyst taught by Hershman may be increased by removing the oxides from the surface of the carbon catalyst before using it in the oxidation reaction. See also, U.S. Pat. No. 4,696,772, which provides a separate discussion by Chou regarding increasing the activity of the carbon catalyst by removing oxides from the surface of the carbon catalyst. Although these processes obviously do not suffer from noble metal leaching, they do tend to produce greater concentrations of formaldehyde by-product when used to catalyze the oxidative cleavage of PMIDA. This formaldehyde by-product is undesirable because it reacts with N-(phosphonomethyl)glycine to produce unwanted by-products (mainly N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as NMG) which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
It has been suggested that the formaldehyde be oxidized to carbon dioxide and water simultaneously with the oxidation of PMIDA to N-(phosphonomethyl)glycine in a single reactor using a noble metal at the surface of a carbon support to catalyze the oxidations, thus giving the following overall reaction:

As the above teachings suggest, in such a process, carbon primarily catalyzes the oxidation of PMIDA to form N-(phosphonomethyl)glycine and formaldehyde and the noble metal primarily catalyzes the oxidation of formaldehyde to formic acid, carbon dioxide and water. Previous attempts to develop a stable noble metal catalyst for such an oxidation process, however, have not been entirely satisfactory.
Like Franz, in U.S. Pat. No. 5,179,228, Ramon et al. teach using a noble metal deposited on the surface of a carbon support. To reduce the problem of leaching (which Ramon et al. report to be as great as 30% noble metal loss per cycle), Ramon et al. teach flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%. Still, the amount of noble metal loss incurred with this method is unacceptable.
Using a different approach, in U.S. Pat. No. 4,582,650, Felthouse teaches using two catalysts: (i) an activated carbon to catalyze oxidation of PMIDA to N-(phosphonomethyl)glycine; and (ii) a co-catalyst to concurrently catalyze the oxidation of formaldehyde to carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl)glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl)glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of PMIDA to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design and employ the two catalysts in a manner so that the oxidation reaction rates are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate that can exceed 10% per cycle.
Ebner et al., in U.S. Pat. No. 6,417,133, describe a deeply reduced noble metal on carbon catalyst which is characterized by a CO desorption of less than 1.2 mmole/g, preferably less than 0.5 mmole/g, when a dry sample of the catalyst, after being heated at a temperature of about 500° C. for about 1 hour in a hydrogen atmosphere and before being exposed to an oxidant following the heating in the hydrogen atmosphere, is heated in a helium atmosphere from about 20° C. to about 900° C. at a rate of about 10° C. per minute, and then at about 900° C. for about 30 minutes. The catalyst is additionally or alternatively characterized as having a ratio of carbon atoms to oxygen atoms of at least about 20:1, preferably at least about 30:1, at the surface as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500° C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere.
The catalysts of U.S. Pat. No. 6,417,133 have proven to be highly advantageous and effective catalysts for the oxidation of PMIDA to N-(phosphonomethyl)glycine and the oxidation of by-product formaldehyde and formic acid to carbon dioxide and water without excessive leaching of noble metal from the carbon support. It has further been discovered that these catalysts are effective in the operation of a continuous process for the production of N-(phosphonomethyl)glycine by oxidation of PMIDA. The advent of continuous processes for the oxidation of PMIDA has created an opportunity for further improvements in catalyst effectiveness (e.g., catalysts that accelerate the rate of oxidation of PMIDA and/or formaldehyde and/or provide improved selectivity). Since the productivity of a continuous oxidation reactor is not constrained by the turnaround cycle of a batch reactor, any improvement in reaction kinetics translates directly into an increase in the rate of product output per unit reactor volume. Furthermore, although the deeply reduced noble metal on carbon catalysts of U.S. Pat. No. 6,417,133 significantly reduce noble metal leaching in acidic oxidation reaction media, further improvements to reduce noble metal losses are nevertheless desirable to improve the economics of the process.